1. Field of the Invention
This invention relates to an electromagnetic trigger device having a trigger circuit for use in cameras, especially cinecameras.
2. Description of the Prior Art
An electromagnetic trigger device for cinecameras is known in which a starting capacitor charged prior to photography is caused to discharge and the discharging current flows to a starting coil, whose magnetic force closes a switch in a drive motor circuit to start photography, whereafter the capacitor completes its discharge to open the switch in the motor circuit, whereby one-frame photography is accomplished.
Such device, however, has offered a problem that one-frame photography fails to properly occur due to variation in the speed of revolutions of the motor, variation in the capacity of the starting capacitor, variation in the resistance of the coil or variation in the source voltage.
Also, in such conventional electromagnetic trigger device, the charge stored in the starting capacitor is discharged as a heavy current through a starting coil of low resistance to thereby start the electromagnetic trigger device and in this connection, the starting capacitor must be of great capacity which in turn leads to a larger size of the device.
Further, U.S. Pat. No. 3,601,481 discloses a system whereby, at the start of photography, a bistable circuit is triggered into conductive state and during the conductive state of the bistable circuit, a current is supplied from a power source to a coil, whose electromagnetic force closes a switch in a motor drive circuit to thereby accomplish photography. Such system requires a current equal to that at the starting to constantly flow to the coil throughout the photography, and this offers some problems including the great power consumption and the necessity of adding a timer circuit for the purpose of one-frame timer photography.